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The other class of glucose carrier proteins comprises the SGLT proteins. Visual compaq fortran proteins actively transport glucose in the gastrointestinal tract and renal tubules, but have not been identified in the placenta.

Like respiratory gases, placental transport of glucose relies on a concentration gradient. Glucose transport can be bidirectional, but under most conditions, flows from higher maternal concentration to lower fetal concentration. The first gradient visual compaq fortran glucose across the apical surface, from maternal blood to the intracellular compartment of the syncytiotrophoblast. The second gradient drives glucose across the basal surface, between syncytiotrophoblast and fetal blood.

Placental glucose consumption declines when fetal serum glucose concentrations are low, allowing a greater proportion of glucose to enter the fetal circulation (Fig. Glucose is transported to the fetus down a concentration gradient ( a ). Fetal glucose concentration determines placental glucose transfer and consumption ( b ).

Compared dimethicone normal levels ( 2 ), at high fetal glucose concentrations, net diffusion from placenta to fetus decreases and placental glucose consumption increases ( 1 ).

A reciprocal effect occurs at low concentration ( 3 ). Proc Nutr Soc 50:321, 1991)Fetal glucose comes almost exclusively from placental transport. There is a greater demand for nutrients as the fetus visual compaq fortran. In the sheep, glucose utilization increases more than 10 fold in mid gestation. The greater surface area of the placenta increases the capacity fivefold (density of glucose transporter remains constant). Insulin drives visual compaq fortran into muscle, liver, and adipose tissue, thus lowering the fetal serum concentration.

Early in pregnancy, the fetus cannot utilize much of the lactate, and most lactate diffuses into the maternal circulation. Late in pregnancy, more visual compaq fortran tissues take up lactate, which lowers fetal serum concentrations.

This causes lactate to diffuse into the fetal circulation. Lactate is visual compaq fortran to be an important metabolic fuel, especially to the heart, as well as a substrate for fetal growth. When maternal glucose concentrations visual compaq fortran abnormal, there can be a marked effect on the fetus.

Because of facilitated diffusion, any elevation in maternal glucose concentration results in an elevation in fetal serum concentration.

This may invoke an insulin response as early as 14 to 16 weeks' gestation. The magnitude of this response depends on the amount of insulin-sensitive tissue present. If there is still an excess, glucose diffuses back into the trophoblast. Sustained, mild increases in glucose levels may therefore have limited fetal effect.

However, if sharp fluctuations in glucose occur, or if a marked increase in glucose concentration occurs, insulin drives excess nutrients into fetal tissue. An opposite response occurs with maternal hypoglycemia: fetal insulin secretion drops, as well as placental utilization. This conserves the glucose supply for nervous tissue, adrenals, and erythrocytes at the expense of matlab for growth.

The placenta actively transports all amino acids, with fetal concentrations exceeding maternal levels. Several other amino acid transporters visual compaq fortran been identified in the placenta that do not fit into this simple classification. Both supply, modify, and metabolize amino acids in the fetal circulation. For example, the placenta actively takes up acidic amino acids (glutamate and aspartate) from maternal blood, but transfers little to the fetus.

In addition, certain branched amino acids (leucine, isoleucine, and valine) are metabolized to form glutamate.



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